Magnetic disk drives conventionally employ feedback control of a travel speed of a magnetic head in a seek operation so as to bring the travel speed of the magnetic head close to a predetermined target speed that is determined in advance according to the remaining distance to a target track. That is, a controller adapted to control the magnetic head speed detects the actual travel speed at predetermined time intervals so as to control the drive current of a voice coil motor based on the difference between the actual and target speeds.
FIG. 7(a) is a graph showing the change of the actual travel speed and target speed of the magnetic head versus seek time. FIG. 7(b) is a graph showing the change of the drive current of the voice coil motor versus seek time. In FIG. 7(a), line A represents the change of the actual travel speed, and line B the change of the target speed. At the beginning of the seek time, the drive current of the voice coil motor rises sharply. In response thereto, the magnetic head accelerates to a predetermined target speed Vtarget1. When the actual speed reaches the target speed Vtarget1, the drive current drops to zero, causing the magnetic head to move at the constant target speed Vtarget1. As the magnetic head approaches the target track, a drive current is supplied to the voice coil motor in the opposite direction to that during acceleration. In response, the magnetic head decelerates to reach the target track.
Incidentally, conventional disk drives described above may produce noise due to the abrupt motion of the magnetic head during seek operation. In the examples shown in FIGS. 7(a) and 7(b), sharp increases in the drive current at the beginning of seek operation leads to abrupt motion of the magnetic head, thus resulting in a high likelihood of noise. Noise may also be produced at the transition from acceleration to constant speed zones (t1 in FIG. 7(a)). Further, at the transition from constant speed zone to deceleration zone (t2 in FIG. 7(a)), the rate of change of the target speed, which is determined to decrease as the remaining distance decreases, is large. As a consequence, the difference (Verror in FIG. 7(a)) between the target speed Vtarget and the actual travel speed becomes temporarily large. This leads to a sharp rise of the drive current of the voice coil motor at this transition, causing an abrupt magnetic head motion and resulting in a high likelihood of noise. In recent years, magnetic disk drives have found increasing application in audio and other equipment for enjoying music and images. Noise during seek operation has become an issue to deal with in these areas.
For example, in this respect, U.S. Pat. No. 5,696,647 (“Patent Document 1”) discloses a magnetic disk drive in which a controller adapted to control a magnetic head outputs a signal, which is adapted to determine a drive current, to a driver circuit of a voice coil motor via a low-pass filter. This suppresses a sharp change in drive current, smoothing the magnetic head motion and reducing noise during seek operation.
However, the magnetic disk drive disclosed in Patent Document 1 exhibits poor control response because the signal from the controller passes through a low-pass filter. This has led to unstable magnetic head speed in the process of transition to constant speed following the termination of acceleration of the magnetic head.